In recent years, as a result of development of miniaturization of large-scale integrated circuit devices (hereinafter, referred to as “LSI”) that are realized by using semiconductors, a discrepancy in the shape or the size between a mask pattern and a processed pattern (e.g., a resist pattern formed by pattern transfer onto a resist film) cannot be ignored any more in a lithography process, which is one process for LSI production.
The miniaturization of a pattern size of LSIs has developed to the extent that the limit of resolution defined by the wavelength of exposure light or the numerical aperture of a projecting optical system of an exposure apparatus has been reached, so that production margin regarding the yield in LSI production, for example, a depth of focus, has been significantly reduced.
When forming a pattern having a desired shape as a resist pattern on a wafer by a conventional pattern forming method, a light-shielding pattern, that is, a mask pattern, having a desired shape is formed on a transparent substrate with a light-shielding film made of a metal such as chromium, and then the wafer coated with a resist film is exposed to light, using the transparent substrate on which the mask pattern is formed as a mask. In this exposure process, a light intensity distribution having a shape similar to the mask pattern formed of the light-shielding film is projected into the resist film. Furthermore, this light intensity distribution generates stored energy in the resist film, and a reaction is effected in a portion of the resist film in which the stored energy becomes larger than a predetermined magnitude. Herein, the light intensity corresponding to the stored energy having a magnitude that causes a reaction in the resist film is referred to as “critical intensity,”.
In the case where, for example, a positive resist is used as the resist film, the portion having a light intensity more than the critical intensity in the resist film is removed by developing the resist film. Thus, a resist pattern having a desired shape can be formed by matching a distribution shape or dimension of the critical intensity value in the light intensity distribution occurring in an exposed material by pattern exposure to a desired pattern.
FIGS. 53(a) to 53(d) are cross-sectional views showing processes in a conventional method for forming a pattern.
First as shown in FIG. 53(a), a film 801 to be processed made of a metal film or an insulating film is formed on a substrate 800, and then as shown in FIG. 53(b), a positive resist film 802 is formed on the film 801 to be processed. Thereafter, as shown in FIG. 53(c), the resist film 802 is irradiated with exposure light 820 via a photomask 810 including a transparent substrate 811 and a mask pattern 812 having a predetermined shape made of a chromium film or the like formed thereon. Thus, a portion corresponding to the mask pattern 812 in the resist film 802 (a portion having a light intensity of not more than the critical intensity) becomes a non-exposed portion 802a, and other portions (portions having a light intensity of the critical intensity or more) become an exposed portion 802b. Thereafter, as shown in FIG. 53(d), a resist pattern 803 constituted by the non-exposed portion 802a is formed by developing the resist film 802.
In the method for forming a pattern as described above, in general, a reduced size projection exposure apparatus is used. The reduced size projection exposure apparatus performs pattern formation by, for example, subjecting a resist film made of a photosensitive resin formed on a wafer serving as a substrate to reduced size projection exposure, using a photomask which is a transparent substrate on which a mask pattern magnified to a size of several times larger than the size of a resist pattern to be formed. In the description of this specification, the following letters are defined as follows:
NA: the numerical aperture of a projecting optical system of an exposure apparatus (e.g., 0.6);
λ: the wavelength of exposure light (light source) (e.g., 0.193 μm)
M: the magnification factor of the exposure apparatus (the inverse number of the reduction ratio, e.g., 4 or 5); and
L: the pattern size (designed value) on a wafer (an exposed material).
For example, in the case where a desired pattern size (designed value) on a wafer is 0.1 μm, L=0.1 μm, and in this case, the mask pattern size on a photomask used in an exposure apparatus having a magnification factor M=4 is 0.1×4=0.4 μm. For simplification of the following description, when indicating a mask pattern size on a photomask, a designed value on a wafer, that is, a calculated value (a value obtained as a result of multiplication by a reduction ratio) is used, unless otherwise specified.
As well known, in the case where light is shielded by a pattern having a size of not more than a half of the wavelength of the light, the contrast of a light-shielded image is reduced by diffraction of light. This means that when the mask pattern is smaller than a half of a value defined by M×λ/NA, where λ is the wavelength of exposure light in a reduction projection optical system, M is the magnification factor, and NA is the numerical aperture, then the contrast of an image transferred by the mask pattern, that is, a light-shielded image is degraded.
FIG. 54(a) shows an example of a layout of the mask pattern 812 on the photomask 810 used in the exposure process shown in FIG. 53(c). As shown in FIG. 54(a), the mask pattern 812 has a size (actual size) of 0.26×M [μm] (M: the magnification factor of an exposure apparatus used in the exposure process).
FIG. 54(b) shows the simulation results of a light intensity distribution projected on the resist film 802 by the photomask 810 shown in FIG. 54(a). The simulation conditions are such that the wavelength λ of the exposure light 820=193 nm, and the numerical aperture NA of the projecting optical system of the exposure apparatus=0.6. In this case, 0.26×M [μm]≈0.8×M×λ/NA. FIG. 54(b) shows a light intensity distribution, using contour lines of relative light intensity (light intensity when the light intensity of exposure light is taken as 1) in a two-dimensional relative coordination system. As shown in 54(b), the light intensity distribution transferred onto the resist film 802 is equal to substantially 0 at a position corresponding to the vicinity of the center of the mask pattern 812. That is, the light shielding properties of the mask pattern 812 is very good.
FIG. 54(c) shows the simulation results of the light intensity distribution along line AA′ of FIG. 54(b), and FIG. 54(d) shows the results of estimating the shape of the resist pattern 803 from the simulation results of the light intensity distribution shown in FIG. 54(b). If the critical intensity is 0.3 as shown in FIG. 54(c), the distribution shape of the critical intensity value in the light intensity distribution shown in FIG. 54(b) is substantially matched to the shape of the mask pattern 812, so that the resist pattern 803 (hatched portion) having a substantially desired shape (shape indicated by a broken line) can be formed, as shown in FIG. 54(d).
FIG. 55(a) shows another example of the layout of the mask pattern 812 on the photomask 810 used in the exposure process shown in FIG. 53(c). As shown in FIG. 55(a), the mask pattern 812 has a size (actual size) of 0.13×M [μm] (M: the magnification factor of an exposure apparatus used in the exposure process).
FIG. 55(b) shows the simulation results of a light intensity distribution projected on the resist film 802 by the photomask 810 shown in FIG. 55(a). The simulation conditions are such that the wavelength λ of the exposure light 820=193 nm; and the numerical aperture NA of the projecting optical system of the exposure apparatus=0.6, which are the same as in the case of FIG. 54(b). In this case, 0.13×M [μm]≈0.4×M×λ/NA. FIG. 55(b) also shows a light intensity distribution using contour lines of relative light intensity in a two-dimensional relative coordination system. As shown in FIG. 55(b), the light intensity distribution transferred onto the resist film 802 reaches a value of about a half of the critical intensity value (0.3) at a position corresponding to the vicinity of the center of the mask pattern 812. That is, the light shielding properties of the mask pattern 812 are deteriorated because of an influence of diffraction of the exposure light 820.
FIG. 55(c) shows the simulation results of the light intensity distribution along line AA′ of FIG. 55(b), and FIG. 55(d) shows the results of estimating the shape of the resist pattern 803 from the simulation results of the light intensity distribution shown in FIG. 55(b). If the critical intensity is 0.3 as shown in FIG. 55(c), the distribution shape of the critical intensity value in the light intensity distribution shown in FIG. 55(b) is not similar to the shape of the mask pattern 812, so that the shape of the resist pattern 803 (hatched portion) is distorted from a desired shape (shape indicated by a broken line), as shown in FIG. 55(d).
Summing up, in the conventional method for forming a pattern shown in FIGS. 53(a) to 53(d), even if a mask pattern is formed with, for example, a complete light-shielding film, it is difficult to form a desired pattern having a size of a half of λ/NA or less, using the mask pattern. Therefore, there is a limitation regarding the size of a resist pattern that can be formed on a wafer.
In order to form a desired pattern having a size of a half of λ/NA or less by emphasizing the contrast of the light intensity distribution generated by a mask pattern, the following method is proposed by H. Y Liu et al (Proc. SPIE, Vol. 3334, p. 2 (1998)): Not only a pattern constituted by a light-shielding film is formed on a transparent substrate as a mask pattern, but also a phase shifter for generating a phase difference of 180° with respect to exposure light between the phase shifter and a light-transmitting portion (a portion on which the mask pattern is not formed) on the transparent substrate is formed. In this method, when the light-transmitting portion and the phase shifter are arranged while sandwiching the pattern (which may be referred to as “light-shielding pattern) constituted by the light-shielding film having a size of a half of λ/NA or less, lights transmitted through the light-transmitting portion and the phase shifter and diffracted to the back side of the light-shielding pattern cancel each other, so that the light shielding properties of the light-shielding pattern can be improved.
Hereinafter, the method of H. Y Liu et al. will be described with reference to the accompanying drawings.
FIG. 56(a) shows an example of the layout of a desired pattern (resist pattern) to be formed. As shown in FIG. 56(a), a pattern 830 has a partial pattern 830a having a size of a half of λ/NA or less.
FIGS. 56(b) and 56(c) show plan views of conventional two photomasks used for forming the pattern shown in FIG. 56(a). As shown in FIG. 56(b), a light-shielding film 842 is formed on a transparent substrate 841 constituting a first photomask 840, and a first opening 843 serving as the light-transmitting portion and a second opening 844 serving as the phase shifter are provided in the light-shielding film 842 while sandwiching a light-shielding pattern 842a for forming the partial pattern 830a. Furthermore, as shown in FIG. 56(c), a light-shielding pattern 852 for forming the pattern 830 (see FIG. 56(a)) in combination with the light-shielding pattern 842a of the first photomask 840 is formed on a transparent substrate 851 constituting a second photomask 850.
The method for forming a pattern using the two photomask shown in FIGS. 56(b) and 56(c) is as follows.
First, a substrate coated with a resist film made of a positive resist is exposed to light, using the first photomask shown in FIG. 56(b). Thereafter, alignment is performed such that a pattern shown in FIG. 56(a) is formed with a latent image formed by the exposure using the first photomask and a latent image to be formed by exposure using the second photomask shown in FIG. 56(c). Thereafter, exposure is performed using the second photomask shown in FIG. 56(c), and then the resist film is developed so as to form a resist pattern. Thus, an unwanted pattern (pattern other than the pattern shown in FIG. 56(a)) that is formed when development is performed after the exposure using the first photomask is removed by the exposure using the second photomask. As a result, a pattern having a size of a half of λ/NA or less can be formed.
In the method of H. Y Liu et al., the contrast of the light-shielded image created by a light-shielding pattern is improved by interposing the light-shielding pattern between the light-transmitting portion and the phase shifter. However, in order to provide this effect, the light-transmitting portion and the phase shifter should be disposed adjacent with a gap of a half of λ/NA or less. In the case where the light-transmitting portion and the phase shifter are disposed continuously on the photomask without interposing the light-shielding pattern, the light intensity corresponding to the boundary of the light-transmitting portion and the phase shifter is smaller than the critical intensity. In other words, a light-shielded image corresponding to the boundary of the light-transmitting portion and the phase shifter is formed. In the case where only a photomask as shown in FIG. 56(b) is used, a light-shielding distribution having an arbitrary shape (distribution of a region having a smaller intensity than the critical intensity in the light intensity distribution) cannot be formed, so that a pattern having an arbitrary shape cannot be formed. As a results, in order to form a pattern having a complicated shape such as a pattern layout of a regular LSI, it is essential to perform exposure using the photomask (second photomask) as shown in FIG. 56(c), in addition to exposure using the photomask (first photomask) as shown in FIG. 56(b). Consequently, the costs of masking is increased, the through-put is reduced because of an increase of the number of processes in lithography, or the production cost is increased.
The method of H. Y Liu et al. has another problem as described below.
FIG. 57(a) shows another example of the layout of a desired pattern (resist pattern) to be formed. As shown in FIG. 57(a), a pattern 860 has a T-shaped partial pattern 860a having a size of a half of λ/NA or less.
FIGS. 57(b) and 56(c) show plan views of conventional two photomasks used for forming the pattern shown in FIG. 57(a). As shown in FIG. 57(b), a light-shielding film 872 is formed on a transparent substrate 871 constituting a first photomask 870, and a first opening 873 serving as the light-transmitting portion and a second opening 874 and a third opening 875 serving as the phase shifters are provided in the light-shielding film 872 while sandwiching a light-shielding pattern 872a for forming the partial pattern 860a. Furthermore, as shown in FIG. 57(c), a light-shielding pattern 882 for forming the pattern 860 (see FIG. 57(a)) in combination with the light-shielding pattern 872a of the first photomask 870 is formed on a transparent substrate 881 constituting a second photomask 880.
However, as shown in FIG. 57(b), in the first photomask 870, a part of the light-shielding pattern 872a is sandwiched between the phase shifters (the second opening 874 and the third opening 875), in other words, the entire light-shielding pattern 872a cannot be provided only between the light-transmitting portion and the phase shifter, which have opposite phases to each other, so that the light-shielding properties of the light-shielding pattern 872a cannot be improved. Thus, there is a limitation regarding the pattern layout that can utilize the effect of the phase shifter.